A diagnostic check offers you a valuable insight into why your car is displaying poor performance, especially if the engine management light is on, giving you the chance to find and fix problems quickly and cost-effectively.
Read on and find out what a car diagnostic test is, how much it costs and why you should book one.
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What Is a Car Diagnostic Check?
A car diagnostic check is a process in which the vehicle’s engine control unit (ECU) uses sensors to identify issues as they occur.
These issues can generate a specific error code which will then be represented on the dashboard via the corresponding warning light.
Though the dashboard can highlight that there is a problem, specialist vehicle diagnostics equipment used by trained technicians is required to read the error code and understand the extent of the issue.
The test can report on the condition of various parts of your vehicle, including the following:
- Engine
- Transmission
- Ignition coils
- Exhaust
- Fuel injectors
- Oil tank
- Airflow
- Coolants
- Throttle
- Brakes
As such, a diagnostic test is an essential step in ensuring your vehicle’s safety and functionality.
If your car is showing signs of deterioration, a diagnostic test can point you in the direction of what needs to be fixed.
How Much Is a Diagnostic Test?
Based on bookings made through our platform between January 2024 and December 2025, the average diagnostic check cost was £49.35.
The cheapest diagnostic check (0-1500cc engine size) booked during this period cost £19.99.
Diagnostic Check Costs by Engine Size
The following average prices are based on bookings made through BookMyGarage between January 2024 and December 2025.
During this period, by engine size, on average a 0-1500cc diagnostic check cost £48.38, a 1501-2400cc test cost £49.77, and a diagnostic check for a 2401cc engine and above cost £54.16.
| Make | Average 0-1500cc Engine | Average 1501-2400cc Engine | Average 2401cc Engine and Above |
|
Audi |
45.24 |
49.63 |
61.43 |
|
BMW |
48.15 |
48.83 |
53.68 |
|
Citroen |
49.18 |
49.07 |
N/A |
|
Fiat |
45.89 |
42.15 |
90 |
|
Ford |
49.27 |
51.31 |
54.42 |
|
Honda |
47.75 |
47.85 |
N/A |
|
Hyundai |
49.08 |
52.06 |
25 |
|
Jaguar |
N/A |
50.14 |
47.19 |
|
Kia |
46.74 |
47.54 |
48 |
|
Land Rover |
53.33 |
50.79 |
54.77 |
|
Mazda |
42.35 |
47.74 |
46.9 |
|
Mercedes-Benz |
46.93 |
50.57 |
58.29 |
|
MINI |
48.77 |
47.85 |
N/A |
|
Mitsubishi |
40.42 |
43.53 |
52.49 |
|
Nissan |
47.18 |
48.19 |
47.25 |
|
Peugeot |
49.43 |
50.55 |
45 |
|
Renault |
48.59 |
47.63 |
N/A |
|
SEAT |
45.81 |
50.4 |
N/A |
|
Skoda |
49.08 |
55.76 |
N/A |
|
Toyota |
43.88 |
46.11 |
48.25 |
|
Vauxhall |
48.46 |
48.13 |
58.3 |
|
Volkswagen |
51.64 |
51.97 |
52.67 |
|
Volvo |
48 |
49.29 |
44.92 |
Diagnostic Check Costs by Location
| Location | Average Cost |
|
Aberdeen |
65.58 |
|
Birmingham |
48.06 |
|
Bradford |
31.31 |
|
Brighton |
38.33 |
|
Bristol |
46.41 |
|
Cardiff |
33.22 |
|
Coventry |
53.34 |
|
Derby |
51.61 |
|
Edinburgh |
45.86 |
|
Glasgow |
42.69 |
|
Hull |
38.24 |
|
Leeds |
50.72 |
|
Leicester |
40.05 |
|
Liverpool |
42.03 |
|
Luton |
70.67 |
|
Manchester |
42.16 |
|
Milton Keynes |
54.23 |
|
Newcastle upon Tyne |
56.35 |
|
Newport |
50.03 |
|
Northampton |
45.44 |
|
Norwich |
45.61 |
|
Nottingham |
45.28 |
|
Plymouth |
45.7 |
|
Portsmouth |
52.4 |
|
Reading |
45.87 |
|
Sheffield |
61.43 |
|
Southampton |
51.83 |
|
Stoke-on-Trent |
40.59 |
|
Swansea |
44.84 |
|
Wolverhampton |
42.9 |
How Do Car Diagnostic Tests Work?
During a car diagnostic test, special software is used which can highlight developing faults that are causing problems with your engine or other systems.
These issues can be detected via built-in processors, sensors and microchips.
The engine control unit (ECU) in your vehicle is where your engine’s sensors can report anything out of the ordinary.
If there is a problem, the ECU will illuminate the relevant light on your dashboard.
When the sensor detects a fault, a code is stored in the car’s computer system.
The mechanic plugs their diagnostic tool into your ECU, allowing them to access the codes and determine which parts need to be repaired.
Once they have fixed the issue, they can clear the fault codes and your ECU will revert back to normal – where it will continue to scan for future issues.
What Can a Car Diagnostic Test Tell You?
A diagnostic test can tell you about a range of issues that your vehicle could be experiencing.
This can include problems such as low engine coolant levels, poor fuel injection system performance, too low or high RPM levels, ignition coil failure or bad ignition timing.
It can also tell you about problems with vital components such as the engine, transmission, exhaust and brakes.
Do I Need to Book a Car Diagnostics Check?
A full plug-in diagnostic check will usually only be required if there is a warning light illuminated on your dashboard - like the engine management light (EML).
An error code or message that appears can also signal that it's time to book a diagnostic test.
How Long Does a Diagnostic Test Take?
A diagnostic test can take anywhere from an hour to 90 mins to complete.
The test can take longer if components need to be removed in order for the check to be carried out.
Is a Car Diagnostic Test Worth It?
The biggest benefits to booking a car diagnostics test are:
- Early detection of problems
- Improved fuel efficiency
- Enhanced vehicle performance
- Emission standards compliance
A diagnostic test can detect problems with your car even before you break down – helping you understand issues such as illuminated dashboard warning lights or unusual noises.
By being proactive and spotting potential issues quickly, you can save yourself money in the long run.
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